The object of this study is to use tetrodotoxin as a molecular probe in studies of the voltage-dependent sodium channel in Drosophila melanogaster. We have isolated a mutant which is abnormally sensitive to orally administered tetrodotoxin. Preliminary genetic experiments suggest this mutant is on the third chromosome. We propose a scheme which will allow us to accurately map the tetrodotoxin sensitivity locus. To characterize the mechanism of action of this mutation we will use electrophysiological recordings from a larval nerve preparation to compare the level of tetrodotoxin required to block nerve conduction in the sensitive Oregon-R 369 strain and the normal Canton-S strain. Studies on the effect of tetrodotoxin on veratridine stimulated uptake of radioactive sodium will be conducted using Drosophila primary neuron and myocyte cultures. We will compare the two strains to determine if the mutant affects the ability of tetrodotoxin to block sodium uptake. Finally, we propose to develop an ultrafiltration assay to quantitate tetrodotoxin binding in Drosophila extracts. This assay will be used in preliminary experiments on the isolation and characterization of the tetrodotoxin binding component of Drosophila.